feat: Implement Step.Gate evaluation (Phase 1: Human Gates)

This implements Phase 1 of the Step.Gate feature (bd-7zka.2):

- bd cook now creates gate issues for steps with gate fields
- Gate issues have type=gate and block the gated step via dependency
- bd list filters out gate issues by default (use --include-gates to show)
- New bd gate command with list and resolve subcommands

Gate types supported in Phase 1:
- human: Manual closure via bd close or bd gate resolve

Implementation details:
- createGateIssue() in cook.go creates gate issues with proper metadata
- collectSteps() creates gate dependencies when processing gated steps
- IssueFilter.ExcludeTypes added to storage layer for type-based filtering
- Gate command provides dedicated UX for gate management

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/sqlite/queries.go

@@ -1719,6 +1719,16 @@ func (s *SQLiteStorage) SearchIssues(ctx context.Context, query string, filter t
 		whereClauses = append(whereClauses, fmt.Sprintf("status NOT IN (%s)", strings.Join(placeholders, ",")))
 	}
 
+	// Type exclusion (for hiding internal types like gates, bd-7zka.2)
+	if len(filter.ExcludeTypes) > 0 {
+		placeholders := make([]string, len(filter.ExcludeTypes))
+		for i, t := range filter.ExcludeTypes {
+			placeholders[i] = "?"
+			args = append(args, string(t))
+		}
+		whereClauses = append(whereClauses, fmt.Sprintf("issue_type NOT IN (%s)", strings.Join(placeholders, ",")))
+	}
+
 	if filter.Priority != nil {
 		whereClauses = append(whereClauses, "priority = ?")
 		args = append(args, *filter.Priority)

internal/storage/sqlite/transaction.go

@@ -1081,6 +1081,16 @@ func (t *sqliteTxStorage) SearchIssues(ctx context.Context, query string, filter
 		whereClauses = append(whereClauses, fmt.Sprintf("status NOT IN (%s)", strings.Join(placeholders, ",")))
 	}
 
+	// Type exclusion (for hiding internal types like gates, bd-7zka.2)
+	if len(filter.ExcludeTypes) > 0 {
+		placeholders := make([]string, len(filter.ExcludeTypes))
+		for i, t := range filter.ExcludeTypes {
+			placeholders[i] = "?"
+			args = append(args, string(t))
+		}
+		whereClauses = append(whereClauses, fmt.Sprintf("issue_type NOT IN (%s)", strings.Join(placeholders, ",")))
+	}
+
 	if filter.Priority != nil {
 		whereClauses = append(whereClauses, "priority = ?")
 		args = append(args, *filter.Priority)